lxd/instance/lxc: Add extra check for devpts_fd
lxd/instance/lxc: Add extra check for devpts_fd

Don't attempt to call the function if we don't have the runtime extension.

Closes #8159

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

stgraber/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd